The invention relates to toxic gas monitors of a type that are useful in continuously monitoring threshold limit values of toxic gases that may be found in certain occupational and industrial environments. More particularly, the invention relates to a pneumatic system for controlling the admission of atmospheric gas samples to a monitoring sensor at levels that are compatible with the linear range of the sensor indicating means. The pneumatic system also includes means for quickly and easily calibrating the sensor and zeroing the indicating meters on a periodically repeatable basis, and further includes convenient means for monitoring a plurality of sampled input areas sequentially and continuously with a single sensor element in the system.
A variety of different types of gas sensors are commercially available for detecting and recording the presence of very low concentrations, i.e., in the range of one-tenth part to two or three parts per million, of toxic gas present in an atmospheric sample. The use of such sensors in underground mines where poisonous gas concentrations are often encountered suddenly due to the release of trapped gas from underground pockets is one example of a common early application for such sensors. In the last few years, the use of such toxic gas monitors has increased sharply due to a growing awareness of long-term health hazards that may be caused by extended exposure to heretofore acceptably low concentrations of certain toxic gases. In fact, various laws have been passed which now specify a range of threshold limit values, commonly referred to as TLV's, that cannot be exceeded for extended time periods in given occupational and industrial environments without subjecting those in control of the area to legal penalties. Examples of such specifications are those promulgated by the National Institute of Safety and Health (NIOSH) under the provisions of the Occupational Safety & Health Act of 1970 that is currently in effect in the United States. The nature of such laws and their associated TLV specifications frequently requires that many working and occupational environments be continuously monitored by very sensitive toxic gas sensors to assure that a suitable warning and/or record of the presence of toxic gases in excess of the TLV's does not occur for undesirably long periods in such monitored environments.
Responsive to this growing need for continuously operable, highly reliable and relatively sensitive gas monitoring apparatus, a number of different types of toxic gas monitoring systems have been developed. While such prior art systems have been found generally suitable for their intended application, they have several disadvantages or drawbacks in common. One of the most common problems encountered with such sensitive gas monitoring sensors is their susceptibility to having associated indicating instruments driven off scale due to unusually heavy concentrations of toxic gases occurring periodically in a sampled atmosphere. In addition to causing loss of accurate qualitative analysis of the samples taken during such intervals when the scale is overdriven, the monitoring sensor may be partially saturated and thus, at least temporarily, rendered inaccurate by the overdose of monitored toxic gas so that it might fail to detect successive samples. A second disadvantage of known prior art sensor systems is that they are normally limited to use in a single sampling location, rather than being readily adaptable to continuously and rapidly monitor a plurality of different environments in sequence.
It has also been found that in order to assure continuous reading precision of the degree often desired on toxic gas monitors that are to be placed in constant industrial environment sampling use, it is desirable to both periodically calibrate the sensor elements of the monitors against a standard gas leak, and periodically zero the sensor instruments by introducing a purified or "zero" gas into the monitoring system. A difficulty frequently encountered when attempting to implement such calibrating procedures on known prior art gas sensors is that considerable time and effort is needed to sufficiently purge the sensor systems of remnants of toxic gases sampled earlier, in order to enable accurate calibration and zero meter setting of the sensors. This same problem of lingering gases in the sensor systems, due to inadequate purging means for quickly and continuously cleansing the systems after each sample is monitored, prevents many prior art sensors from being readily adaptable to rapidly and sequentially monitor a plurality of different atmospheric environments.